Following a somewhat recent introduction, optical fiber has experienced a meteoric rise as the predominant means of transmission media in voice and data communications. Typically, an optical fiber has a diameter on the order of 125 microns, for example, and is covered with a coating material which increases the outer diameter of the coated fiber to about 250 microns, for example. Coated optical fibers typically are assembled into units or ribbons disposed within a tubular member and enclosed in a sheath system which may take any of several forms.
Because of the increasing number of optical fiber cable miles being installed in recent years, an increasing number of underwater installations have been necessary. In many instances, these so-called water crossings have been made to effect the most economical right-of-way acquisition. Water crossings include cables which are routed across rivers, lakes and bays, for example.
Typically, an underwater cable includes a core portion comprising a plurality of optical fibers which may be in ribbon form. The core portion may comprise a complete terrestrial optical fiber cable or protective layers of plastic jacketing and/or strength members. Further, the core portion is enclosed by an outer sheath portion which often is referred to as an oversheath and which includes a plurality of layers of metallic strength members and twine bedding layers and a tar-impregnated twine outer protective wrap, for example. It has been conventional practice for the strength members of the outer sheath portion of the cable to be made of galvanized steel wire strand material. By core portion is meant the portion of the cable interior of the strength members and bedding layers of the outer sheath portion.
The layers of the strength members serve two useful purposes. First, because of anticipated marine traffic, armoring is required for mechanical protection. Also, the strength members provide sufficient weight to cause the cable to rest securely on the bottom of the body of water through which it is routed.
When an underwater cable is deployed, water enters the twine layer and migrates to the interstices of the cable armoring wires because the tar-like coating on the outer protective twine wrap does not form an integral water blocking sheath. With the water in contact with the strength members, hydrogen generation may occur.
Hydrogen-induced attenuation at both the 1310 nm and 1550 nm operating wavelengths has been observed in installed, armored underwater optical fiber cables which include the conventional galvanized wire strength members. Studies have shown that even when the core portion by itself does not generate hydrogen, added loss can occur.
Hydrogen is generated by one of two mechanisms or both. The first way is through self-corrosion of a metal. All metals, except perhaps the noble metals, e.g. gold and platinum, have some finite corrosion rate in natural environments. When a metal corrodes, the surface is covered by micro/macroscopic cells where an anodic (oxidation) reaction occurs, i.e. corrosion of the metal, but the surface also is covered by micro/macroscopic cells where a cathodic (reduction) reaction takes place. In acidic and neutral waters this cathodic reaction can produce deleterious hydrogen molecules. In order for corrosion to occur, a cathodic reaction must occur to consume the electrons liberated in a corrosion reaction, otherwise the corrosion reaction cannot take place. Metals characterized by a relatively high chemical or electrochemical reactivity are referred to as active metals, and will be more likely to produce hydrogen than a metal characterized by a relatively low chemical or electrochemical reactivity.
The second way in which hydrogen can be generated in cables is through electrical disturbances. These disturbances can be caused by galvanic cells which set up between involved metals, by electrical cells such as long cells in which the anode and cathode are separated by a relatively long distance and which set up naturally by the environment, by corrosion mitigation measures such as cathodic protection used on the cable itself, and by interference from other cathodic protection systems or from stray currents picked up from power ground return arrangements.
An electrical disturbance can induce currents on a cable and cause that cable either to corrode or to generate hydrogen. To avoid corrosion, the cable can be made cathodic by using a cathodic protection system. However, this also leads to the generation of hydrogen.
In accordance with today's government regulations, any polluting body is required to have environmental protection. An anode corrodes and is consumed. An underwater pipeline, cable, mothballed ship, bridge support or pier, for example, may be anodic and corrode. In order to prevent corrosion of the above structures, the corrosion process is reversed by applying cathodic protection to that structure. The structure is then made cathodic to an anode placed nearby in the water or ground. Due to the proximity of the cable to the cathodically protected structure or due to the electrical bonding, i.e. grounding, of the cable and the structure, currents can be induced in the cable causing the cable to produce hydrogen and/or corrode. Mitigative measures to obviate cathodic protection interference involve electrical bonding to make the cable cathodic like the interfering structure and produce hydrogen, or applying an additional cathodic protection system to the cable which would make the cable separately cathodic, but also producing hydrogen.
An electrical disturbance also can appear as electrical currents picked up by the cable which can present itself as a lower resistance ground path for an electrical power substation return. These currents can emanate from direct current traction systems, from substantial welding activity in shipyards, or from poorly grounded electrical equipment. The lengths of cable involved in power pickup would produce the hydrogen while the lengths involved in power loss would experience corrosion. Mitigation involves either electrical bonding at the power loss point or the use of a cathodic protection system on the cable, causing that cable to be cathodic.
Further, it has been found that electrical disturbances can lead to the generation of hydrogen in cables routed in fresh water as well as in sea water. Still further, electrical disturbances will cause hydrogen generation notwithstanding the use of strength members which are not active metals in the outer sheath system. By a metal which is not active is meant one which does not have a relatively high chemical or electrochemical reactivity. Even worse, the hydrogen generation may proceed whether a driving voltage is relatively low as well as relatively high.
The problem of electrical disturbances is not uncommon. For example, when a cable is routed into a manhole, exposed metals must be grounded to the grounding system in the manhole. Hence, cathodic protection in flooded manholes is commonly provided. Cathodic protection of telephone plant is standard. Further, it has been found that in some bodies of water, an electrical potential exists between different portions of the water, thereby facilitating the generation of hydrogen from cathodic structures which extend therethrough.
It had been thought that because the outer sheathing of the cable is comprised of twine with an application of tar-like material to the twine, any hydrogen which was generated would migrate out from the cable and thus no added loss would occur. This has been found to be an incorrect assumption.
Unfortunately, the hydrogen does not diffuse readily or completely out of the cable. Water in the vicinity of galvanized wire outer strength members interacts with the zinc coated, i.e. galvanized, wire and causes corrosion. Corrosion causes hydrogen gas to be given off. The hydrogen that is generated inside the cable occupies the interstices in the cable. Bubbles form, but these may be microscopic in size. The only way for the hydrogen to escape from the cable is as relatively large bubbles. Although the twine-tar layer allows water to enter the cable, it does not readily allow the hydrogen bubbles to coalesce into bubbles which are sufficiently large to escape the cable unless the rate of hydrogen generation is very high. Because hydrogen is not soluble in water, it remains in the cable. Enough remains until the outside pressure is overcome. At 32 feet of water, an additional atmosphere is required to overcome the water pressure.
Diffusion is a partial pressure (concentration) driven effect. The partial pressure of hydrogen in the optical fiber cable core is essentially zero, i.e. 10.sup.-6 atmosphere ambient, and hydrogen cannot diffuse into the water because it is not soluble there. As a result, the hydrogen diffuses into the cable core, and hence into the fibers until the partial pressure inside the optical fiber cable equates to that in the outer twine layer. The magnitude of partial pressure will show a depth dependance because the concentration (molecules/cc or partial pressure) will be greater with increasing depth. Hence, added loss in the optical fiber will be in direct proportion to the depth of the cable in the water.
The prior art has recognized the problem of hydrogen generation in cables through the mechanism of corrosion. In one prior art patent, GB 2145536, an underwater optical fiber cable includes a central tension-resisting member, a pressure-tight structure which is made of aluminum and tension-resisting members. In order to avoid degradation of the transmission medium, a number of solutions are proposed in that patent. The central tension-resisting member may be made of non-metallic material, or a pressure-tight structure is formed of copper and the central tension-resisting member is formed of copper or a non-metallic material. Metals of low chemical or electrochemical reactivity can be selected. Another solution cable includes the use of similar metals to prevent the generation of a galvanic cell. A fourth embodiment includes metallic coatings on all metallic members or those of strong chemical or electrochemical reactive forces. Another embodiment features all metallic members or at least those of high chemical or electrochemical reactivity be given a non-metallic coating.
It has been found that there are shortcomings to the solutions of the prior art. Although strength members which comprise a material of high chemical or electrochemical reactivity are coated with a non-metallic material, it has been found that under certain conditions, degradation of the transmission medium still may occur because of the diffusion of hydrogen into the core. The sought-after solution cable must offer protection for the optical fiber beyond that provided by prior art cables. Otherwise, optical fiber cables installed underwater, be it fresh or sea water, will most probably continue to experience unacceptable added loss.
None of the prior art solutions appear to address the problem of electrical disturbances. This is understandable inasmuch as this second mechanism for the generation of hydrogen which has been described earlier herein has not been recognized by the art. This potential problem caused by electrical disturbances may have been overlooked by those skilled in the art because of too much focus on cable corrosion only. What seemingly was not considered were the environment and external environmental influences.
What is needed and what seemingly is not provided for in the prior art is an underwater optical fiber cable which prevents hydrogen generation to avoid degradation of the transmission medium of the cable. Desired is a cable structure that overcomes not only the problem of hydrogen generated bt corrosion but also that caused by electrical disturbances. The sought-after protective structure should be adaptable to a number of different cable core portions. Many presently used cable designs integrate the strength member system which is required for underwater use with the core and with other elements of the sheath system of an otherwise terrestrial cable. What also is desired is a cable structure in which a non-hydrogen generating strength member system comprises a portion of an oversheath which may be used with any number of different core portions.